Field of the Invention
The present invention relates to an image capturing apparatus using an image pickup device having a focus detection function based on a phase difference method.
Description of the Related Art
A contrast detection method has conventionally been known as a general method using a light beam having passed through an imaging lens in an automatic focus detection/adjustment method for an image capturing apparatus. This method uses an image pickup device as a focus detection sensor, and can adjust the focus by evaluating an output signal from the image pickup device and moving the focus lens position to maximize the contrast. However, contrast information needs to be evaluated while moving the imaging lens. After it is detected as the evaluation result that the contrast is maximum, the focus lens needs to be moved again to the maximum-contrast position. Therefore, focus detection takes time, and a high-speed operation is difficult.
To solve this problem, there is proposed a technique capable of directly obtaining the focus shift amount of an imaging lens while using an image pickup device as a focus detection element by incorporating the phase difference detection function in the image pickup device.
For example, in Japanese Patent Laid-Open No. 2000-156823, a pupil division function is given to some light receiving elements in an image pickup device by decentering the sensitive region of a light receiving unit from the optical axis of an on-chip microlens. These pixels are arranged in the image pickup device at a predetermined interval, implementing the phase difference detection function.
Japanese Patent Laid-Open No. 2010-219958 discloses the following technique: focus detection pixels are arranged on rows which are thinned out in thinning readout. When displaying a moving image, the thinned-out rows are read out and displayed to read out rows necessary for moving image display. Further, vertical scanning is performed to read out rows on which focus detection pixels are arranged in the same frame. In addition, the accumulation time is switched between the rows necessary for moving image display, and the rows on which the focus detection pixels are arranged. Accordingly, accumulation control is performed to achieve proper exposures on these rows. By this control, a high S/N ratio of the focus detection pixels can also be ensured.
However, the conventional technique disclosed in Japanese Patent Laid-Open No. 2000-156823 described above has three readout modes: a still image mode in which all pixels are read out, a thinning readout mode in which rows each including only imaging pixels are read out, and a distance measurement readout mode in which only focus detection pixels are read out. In an electronic viewfinder mode and moving image mode, the frame rate of a moving image can be increased by thinning readout. However, since no focus detection pixel is read out, high-speed focus detection using the phase difference method cannot be performed.
The conventional technique disclosed in Japanese Patent Laid-Open No. 2010-219958 discloses vertical interlaced scanning out of readout methods of reading out pixels while thinning them out. When generating a moving image from a multi-pixel image pickup device optimized for recent still images, moving image data are preferably acquired at various thinning rates. However, this has not been considered so far.
For example, a mode in which the thinning rate is relatively low to obtain high-resolution moving image data having a large number of recording pixels, and a mode in which the thinning rate is relatively high to obtain moving image data having a high frame rate will be examined. In this case, if the same thinning readout as that for imaging pixels is performed on rows on which focus detection pixels are arranged, outputs from the intentionally arranged focus detection pixels may be thinned out and may not be read out depending on the arrangement density of the phase difference detection pixels.